The method and apparatus of the present invention relate to the operation of pinch rolls of the type utilized to facilitate the withdrawal of a strip of material from a rotatable mandrel or the like for subsequent processing. More particularly, the present invention relates to an improved system for at least partially operating a pair of pinch rolls by means of a hydrostatic drive unit wherein relatively low pinching forces are utilized to withdraw the strip material from the mandrel.
Continuous strip material, such as steel, is typically provided in tightly wound coils which are loaded onto a mandrel for subsequent delivery to various types of operational lines for processing. For example, it may be desired to cut the continuously coiled strip into flat sections of predetermined length prior to shipment of the steel to a customer. This is conventionally accomplished by uncoiling the strip from the mandrel and feeding it through a precisely controllable power shear such as is found in the well known Hallden shear line. It will, however, be appreciated that the accuracy with which the shearing operation is performed will be largely dependent upon the accuracy with which the strip material is itself delivered to the shear. That is, in order to achieve a repeatably accurate cut, the strip material must be delivered to the shear at some preselected speed which can be maintained substantially invariable.
Conventionally, the delivery of strip material, in particular steel, from a rotatable mandrel to a processing station, such as a power shear, is facilitated through the use of a pair of vertically disposed pinch rolls positioned between the supply mandrel and the processing station. The strip material is advanced through the closely spaced pinch rolls by means of frictional forces which result from the application of downward pressure, generally provided by a biased spring, on the top roll and the torque output from the bottom roll which is typically driven by an electric motor or the like. It is well known in the art that, when utilizing the aforementioned technique, relatively large pinching forces must be exerted on the strip by the rolls. In other words, in order to maintain the tangential velocity of the bottom driven roll and the strip speed equalized, the downward normal force exerted on the top roll must be quite high. These high frictional forces tend to induce increased roll wear and limit the useful life of the rolls.
In addition, to further increase the efficiency of prior art pinch rolls, it has been relatively common practice to employ patterned rolls, such as the spiral grooved rolls disclosed in U.S. Pat. No. 3,771,703 issued to St. Denis on Nov. 13, 1973, to increase the gripping forces exerted by the rolls on the strip. However, this approach frequently results in an undesirable roll pattern transfer to the strip material which is especially noticeable in cases where higher level pinching forces are required.
Moreover, due to the aforementioned method of operating prior art pinch rolls, slippage between the rolls and the strip material is a commonly experienced problem which is particularly encountered when running heavily oiled strip material through the rolls. It will be appreciated that such slippage will affect the rate of delivery of the strip to the processing station and, for example, in the case of a shearing operation, will result in irregularly sheared strip sections.
While the above discussion relating to prior art has, at times, referred to certain specific apparatus, it will be appreciated that it is known to use various equivalent devices in lieu thereof, although such use has not cured the various deficiencies previously discussed herein. For example, the previously mentioned St. Denis patent discloses the use of hydraulic means instead of an electrical motor for driving the bottom roll. Also, it is known to provide a downward normal force on the top roll through the agency of hydraulic means instead of springs. See for example, U.S. Pat. No. 3,349,981 issued to H. F. Hawkins et al, on Oct. 31, 1967. Therefore, although somewhat different means are utilized, the general prior art operating scheme of employing the pinching force-friction method of enslaving the top roll with a downward normal force to the driven bottom roll is retained.